Compare Luther's explanation in the Large Catechism (I recommend reading the whole thing some time, in his explanation of the 10 commandments):
This commandment, therefore, according to its gross sense, does not concern us Christians; for it is altogether an external matter, like other ordinances of the Old Testament, which were attached to particular customs, persons, times, and places, and now have been made free through Christ.
So, without going into incredible detail, it appears that the Sabbath becomes another day of work, holy and sanctified work. Somehow this is ironic coming form the great proclaimer of justification by faith alone. Maybe it is because Luther was a workaholic? (now I'm sounding like Erikson the psychologist).
Anyway, what I noticed this time around as I prepare to preach on Sabbath, is that on the other six days of creation, God "spoke", and there was. God said, let there be... Our creeds affirm that this word spoken by God was the Word, Jesus Christ, and so we can confess in the creed that "all things came into being through Him."
But on the 7th day, God does not speak. God rests. And if God rests, then God does not speak, and so also the Word is not enunciated. Sabbath is a quiet day, at least as it is described in Genesis. God does not speak. The Word does not resound. The Spirit does not hover. Instead, God blesses and hallows the day.
So is there something quirky about having made Sabbath a Word day? Not necessarily, for Jesus is Lord of the Sabbath. But Sabbath for Christians is about resurrection. It is about sitting down and resting and celebrating the great abundance of God's good creation. God's creation is especially clear to us because in Christ there is new creation.
But this probably means Sabbath is about more than keeping God's word holy. It also means ceasing from work. It means especially ceasing from consumption, fro living out of the gospel of scarcity. It means living a full day in the abundance of God, where God provides everything we need.
Maybe a better place to learn the Sabbath from Luther in his explanation of the creeds, where he says, using the word "abundantly" abundantly peppered throughout, "God created me and all that exists... God preserves... God provides... God protects... Jesus has redeemed... the Holy Spirit has called me through the gospel... enlightened me... made me holy... the Holy Spirit forgives all sins... the Holy Spirit will raise us up on the last day."